Break up the oreos into chunks. I personally prefer larger pieces but if you don't a food processor could be used.
Prepare an 8x8 pan with parchment, but also greasing (unless non stick). Set aside
In a small bowl, measure out ⅓ of the can of sweetened condensed milk (about 130g) and in a medium bowl pour the rest of the condensed milk (about 260g).
Break up the semi-sweet chocolate bar into the smaller bowl of condensed milk, and the white chocolate into the larger bowl.
Either in a double boiler or the microwave, melt the chocolate into the condensed milk. If melting by microwave, stop and stir about every 30 seconds. You will want to do this so both bowls are ready at roughly the same time.
Working quickly, pour the two bowls into the prepared pan. I found it easiest to pour all the white chocolate in and then pour the chocolate in patches around the white chocolate.
Add the crushed oreos to the fudge.
Using a knife, swirl the two flavors together, mixing the oreo into it as well creating a marbled fudge.
Cover and let fudge sit for at least 12 hours before serving. Cut and store in an airtight container.
